Thanks to higher spatial reuse of spectrum, short-radius cells (further shortened as picocells) appear as a promising 4th Generation (4G) solution to satisfy bandwidth extensive traffic demands and to enhance the Quality of Experience (QoE) of mobile users.
However, operating high density of picocells raises major issues not only in terms of radio interference but also in terms of mobility management. Given the short distance separating neighbouring picocells, high speed users are expected to cross a large number of picocells in a short time frame. Indeed, the handover rate increases with the user speed and with the inverse of the inter-cell distance.
In this context, guaranteeing a steady QoE to high speed users moving through picocells becomes challenging: cell reselection, Radio Resource Control (RRC) reconfiguration, attachment to the new cell have to be performed in a very short time to maintain call connection and to avoid Radio Link Failures (RLF).
The known solutions to this problem are:                [Fast HO] The handover procedure is speed up to ensure rapid re-attachment (cell reselection, authorizations, handover preparation) to the most suitable target cell, while preventing unreliable handover attempts.        [Umbrella Cells] High-speed users are handled by umbrella macro cells.        [VC] Multiples picocells are combined into one single aggregated virtual Cell (VC).        
Fast HO encompasses any mechanism that accelerates the HO, such as scanning optimization duration as described in the European patent application EP2207382 entitled “Method for Prioritizing Handover Targets for scanning by a mobile Terminal in a wireless Network”, or optimization of Time-To-Trigger events.
However, it proves inefficient at high speeds. Indeed, HO duration can amount up to 1 second. At 90 km/h, the user would have travelled about 25 meters, which is a very long distance when compared to typical cell radius of pico cells (about 100 meters).
Added to this, physical layer measurements can be critically affected by distance dependent fading, shadowing, etc. So, handover measurements and Time-To-Trigger (TTT) handover have to account for these radio fluctuations and cannot be arbitrarily reduced.
The handover decision process can also account for user speed information. The idea is to re-direct fast-moving users towards overlaying macro cells, referred to as umbrella cells, while slow-moving users keep on being advantageously handled by picocells. An umbrella cell is operated by a distinct macro base station, and has a radio coverage area that encompasses many picocells.
The concept of virtual cells (VC) was first described in the article entitled “Handoff in Virtual Cells System based on Distributed antenna” from Y. Mo, J. Xie and B. Huang, published by Wicom in 2006. VC are a set of cooperating radio cells and radio base stations that are seen by the mobile as a single distributed base station. Inside the VC, the mobile can move across the cells without performing handovers. Handover is only performed at VC boundaries. In this way, handover rate is significantly reduced.
VC based handover solution relies on distributed architectures with distributed antennas structure. Added to this, implementation complexity arises from the construction and selection of the VC.